Method for producing nanosilica plates

ABSTRACT

The present invention relates to an exfoliating agent and to a process for producing random form of nanoscale silica plates. Two types of exfoliating agents are applied in the present invention, which respectively have the formula:  
                 
 
                 
wherein R is a polyoxypropylene group, polyoxyethylene/oxypropylene group, or polyethylene amino group. In this invention, layered silicate clays are exfoliated into random silica plates by acidifying AMO or AEO with inorganic acid, adding the acidified AMO or AEO to layered silicate clay with agitation, and adding sodium hydroxide or chloride of alkali metal or alkaline-earth metal, in ethanol, water and a hydrophobic organic solvent to the intermediate product and repeating phase separation procedures to isolate random silica plates from water phase.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the use of polymeric polyamines asexfoliating agents for producing random nanosilica plates from layeredsilicate clays, and particularly to a method for producing nanometerscale silica plates by directly exfoliating inorganic layered silicateclays with multiple functional quaternary salts, which are preparedspecifically from amine-terminating Mannich oligomers (AMO) oramine-terminating epoxy oligomers (AEO). The invention also relates to amethod involving ionic exchange between layered silicates and organicquaternary amines, aqueous sodium hydroxide extraction process,recycling the exfoliating agents and finally the isolation of randomsilica plates.

2. Description of the Related Art

Currently, inorganic/organic polymer composite materials in a nanometerscale regime (approximately 1˜100 nanometer scale) are one of the mostimportant materials, and thus have been widely investigated anddeveloped. Such nanocomposite materials have two different phasesincluding inorganic and organic components, wherein at least one phaseis dispersed under a nanoscale regime in a homogeneous manner.Accordingly, the compatibility in the nanoscale mixing between twodistinct phases, for example, inorganic clay and organic polymer, is theessential factor for the nanocomposite's physical and mechanicalperformance. In general, the clay/polymer hybrid materials can beclassified into two categories of composites, the intercalated and theexfoliated, on the basis of the clay dispersion in polymer matrix. In anintercalated structure, the silica plates still maintain their layeredstructure but with the addition of organic intercalants anchored in thegallery. In the exfoliated form, each individual silica plate israndomly dispersed in the polymer matrix. Dramatically improved physicalproperties are often obtained for the exfoliated structure, asdemonstrated by the first commercialized Nylon6/montmorillonitenanocomposites. In such a system, the silica plates (about 100×100×1 nmin dimension) are well-entangled with polymer strands through van derWaals forces and evenly distributed in the polymer matrix. Only a lowpercentage of exfoliated silica plates is required to enormously improvethe mechanical and physical properties of the blended material.

Since the naturally occurring layered silicate clays are hydrophilic,the dispersion of clay in polymer matrix on a nanometer scale is ageneral problem. The process requires an enlargement of the clayinterlayer distances by means of organic quaternary ammoniumincorporation, and thus monomers can enter into the clay interfacethrough ionic exchange reactions. The monomers can then be polymerizedwithin the interlayer space to obtain an exfoliated inorganic/organicpolymer nanocomposite material. In principle, the enlarged distance ispreferred to be wide enough for monomer or polymer molecules to enter.After exfoliation, the layered structure is randomized into irregularshapes and the silica plates have different directions without anycrystalline form. The random silica plates are therefore dispersed inorganic polymers as nanocomposites.

Conventional intercalating agents such as 12-aminolauric acid,hexadecylamine, fatty amine, bis(2-hydroxyethyl) methyl tallow alkylamine and stearylamine, usually have low molecular weights and can beconverted into the corresponding ammonium salts such as quaternaryammonium chloride salt. Through ionic exchange reactions, the counterions in interlayer spaces of the clay can be ionically exchanged andhence the interlayer distance expanded to a certain degree.

Referring to the research of T. J. Pinnavaia (Michigan StateUniversity), intercalating agent CH₃(CH₂)_(n)—NH₃ ⁺ is provided toexchange with metal ion salts within the layer-structuredmontmorillonite clay (MMT) in preparing intercalated and organicmodified MMT, which is then dispersed in diglycidyl ether of bisphenol-A(such as epoxy resin Shell Epon 828) to form a epoxy polymer-claycomposite material in a nanoscale dispersion. By using suchintercalating agents, the interlayer distances of MMT can be enlarged to18.0 Å. The epoxy resin can then enter into the interlayer and form anepoxy/clay material through curing polymerization at 75° C. Thisreference also indicates an improvement in heat distortion temperature.The intercalating agent performs a role of monolayer to bilayer, andeven to pseudo-trilayer. The interlayer distance ranges between13.8-18.0 Å, which allow the epoxy resin to polymerize therein, andfurther to exfoliate the layered inorganic matter so that performanceadvantages are achieved.

Japanese Patent No. 8-22946 (Toyota Company) discloses the firstcommercial inorganic/organic polymer composite material under ananoscale regime. This composite material is produced by dispersing[H₃N⁺(CH₂)₁₁COO⁻]-montmorillonite in Nylon 6, wherein theaminocarboxylic acid is provided as an intercalating agent and thepolymers are formed between layers of the aminoacid intercalated claythrough condensation of caprolactam monomers to Nylon 6 polymer. In thisinvention, since the aminocarboxylic acid intercalating agent ishydrophilic, the modified clay is suitable for Nylon 6 compatibility butcan not easily mix with nonpolar polymers such as polyethylene andpolypropylene in an uniform manner. Accordingly, Japanese PatentPublication No. 8-53572 provides other organic onium ions as anintercalating agent to mix with layered silicate, which can be uniformlydispersed in molten polyolefin resin. Unfortunately, the organic oniumions can only enlarge the interlayer distances to a certain degree andthe affinity between the intercalating agent and the polyolefin resin istoo weak to exfoliate the layered silicates.

In general, the difficulty for the exfoliation by using conventionalquaternary ammonium salt is caused by the inherent chemical structure ofthe clay. The chemical structure of the common smectite clays such asmontmorillonite consists of ionic pairs of ≡Si—O⁻ anions on the surfaceand counter metal cations. The surface ionic charge interaction tightlybinds the neighboring silica plates together and maintains the primarystacking structure. The exfoliation of the layered silica plates hencerequires a tremendous force to overcome the inherent ionic bridges.Moreover, the conventional process requires two separate steps. Thelayered silicates are first ionically exchanged with an intercalatingagent such as amino acid, or alkyl ammonium quaternary salt. Theintercalated silicates at this stage are embedded with organic salts andthe gallery distance is widened to a commonly 18-40 Å. In the presenceof organic intercalants, the modified clays become organophilic and arepossibly exfoliated. Accordingly, there is a need to ameliorate theprocess by means of providing appropriate intercalating agents andoperation conditions which may exfoliate the silicate clay directly forbetter compatibility with other polymer materials. In addition, it iseven more desired to prepare the exfoliated form of silica plates whichare free of organic polymers, so that the random silica plates in pureform and free of organic contaminations can be mixed with differenttarget polymers to produce improved properties without encountering thedispersion problem in the process.

BRIEF SUMMARY OF THE INVENTION

The main object of the present invention is to provide a method forproducing random nanoscale silica plates, in which the layered claystructure may be exfoliated directly by an exfoliating agent to producerandom nanosilica plates.

Another object of the present invention is to provide a method forproducing random nanosilica plates, whereby the organic exfoliatingagent can be recycled through sodium ion exchange and the random silicaplates may be isolated in water suspension without organiccontamination.

The present invention involves a multiple-step process: (a) acidifyingthe exfoliating agent with an inorganic acid to form a quaternaryammonium salt; (b) mixing the acidified exfoliating agent with a swelledinorganic silicate clay so as to exfoliate the clay directly throughcationic exchange reaction; (c) adding an aqueous solution containing ahydroxide or a chloride of alkali metal or alkaline-earth metal,ethanol, water and an organic solvent, and then keeping the mixturestatic to form an upper organic phase and a lower water phase containingthe nanosilica plates, and isolating the lower water phase. The randomnanosilica plates, dispersed in the water phase, contain no impurity andtherefore are suitable for further application in modifying polymers.

In order to achieve the above objects, the process of the presentinvention requires the preparation of the novel exfoliating agentsoptionally from one of the following synthesis methods: (1) preparingamine-terminated Mannich oligomers (AMO) from polyoxyalkylene amine,p-cresol and formaldehyde at a molar ratio of (n+1):n:2n, where n=1 to5, the AMO having a general formula:

(2) preparing amine-terminated epoxy oligomers (AEO) from the epoxyopening reaction of polyoxyalkylene amine and diglycidyl ether ofbisphenol-A at a molar ratio of (m+1): m, where m=1 to 5, the AEO havingthe formula:

wherein the polyoxyalklene amines are diamines of the formula:

where x=2 to 120 or or the molecular weight is in the range of 200 to8,000.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the steps of the methodproducing nanosilica plates in this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method for producing random nanoscale silica plates of the presentinvention can be exemplified by the following process block scheme,illustrating a general process starting with AMO or AEO. The overallprocess comprises (1) acidifying the exfoliating agent with an inorganicacid to form quaternary ammonium salt; (2) mixing the acidifiedexfoliating agent with a swelled inorganic silicate clay so as toexfoliate the clay directly through cationic exchange reaction; (3)adding an aqueous solution containing a hydroxide or a chloride ofalkali metal or alkaline-earth metal, ethanol, water and an organicsolvent, and then keeping the mixture static to form an upper organicphase and a lower water phase containing the nanosilica plates, andisolating the lower water phase.

The following steps are illustrated in FIG. 1. In step 1, the polyamineexfoliating agent is pretreated with a controlled amount of inorganicacid to produce quaternary ammonium salts in water emulsion. Theequivalent ratio of the exfoliating agent to the inorganic acid ispreferably 2:1 with respect to amine equivalent to acid, wherein theinorganic acid can be hydrochloric acid, sulfuric acid, phosphoric acid,nitric acid, etc.

In step 2, the acidified polyamine exfoliating agent, in the form ofquaternary ammonium salt with multiple cation reactive sites, is addedto layered silicate clay, which is pretreated with water for swelling.In the following step, the inorganic clay is added with vigorousagitation to facilitate the ionic exchange reaction between thequaternary salts and the sodium ions of the clay. The cationic exchangeequivalent ratio of the exfoliating agent to the silicate clay ispreferably 1:1 or larger with respect to acid equivalent to silicatecationic exchange capacity.

The clay is selected from one of the following naturally occurringsmectite clays, including montmorillonite, kaolin, mica, layered doublehydroxide, (J. H. Choy, S. Y. Kwak, Y. J. Jeong, J. S. Park, Angew.Chem. Int. Ed. 39, 4041(2000), M. Templin, A. Franck, A. D. Chesne, H.Leist, Y. Zhang, R. Ulrich, V. Schadler, U. Wiesner, Science 278, 1795(1997), M. B. Armand, in Polymer electrolyte reviews-1 (eds J. R.Maccallum, C. A. Vincent,) (Elsevier applied science, New York andLondon, 1987), etc., and preferably has a cationic exchange capacityranging from 50 meq/100 g to 200 meq/100 g. Among the inorganic layeredsilicate clay, sodium montmorillonite (Na⁺-MMT) is used in the preferredembodiment. The clay may be commercially purchased from Nanocor,Southern Clay, Kunimine Industries Co. The structure is composed ofprimary structure containing 8-10 layers of lamellae silicate as thefundamental unit and secondary structure containing the aggregates ofprimary units. In the primary units, each layer of the lamellae is about9.6-10 Å thick and the interlayer distance is about 12 Å. The size ofthe silicate aggregates usually ranges form 0.1μ to 10μ, which isrequired to be swelled with water to form a slurry beforehand.

In step 3, the hydroxide or chloride of alkali metal or alkaline-earthmetal is preferably sodium hydroxide, and added at the same equivalence.More preferred alkali salts are sodium hydroxide, potassium hydroxide,lithium hydroxide, cesium hydroxide, calcium hydroxide, magnesiumhydroxide, aluminum hydroxide and their mixtures or derivatives. Thealkali compound provides alkalinity to neutralize or convert thequaternary ammonium salt to amine. Consequently, the emulsion is brokenand a two-phase solution is formed. The alkaline salt is preferablyadded in excess amount, for example, at double or triple equivalence tothe original clay CEC value. The organic phase formed in step 3 containsthe AMO or AEO exfoliating agents which can be easily phase separatedfrom the water phase and then recycled.

In this step, the organic solvent can be alcohol, ether, ketone, ester,nitrile, saturated hydrocarbon, chlorinated hydrocarbon or aromatichydrocarbon; for example, methanol, ethanol, isopropanol,tetrahydrofuran, diethyl ether, di-isopropyl ether, methyl tert-butylether, ethyleneglycol dimethylether, dipropyleneglycol monomethyl ether,methyl isobutyl ketone, acetonitrile, ethylnitrile, phenylnitrile,benzylnitrile, ethyl acetate, methyl acetate, pentane, hexane, heptane,cyclohexane, chloroform, dichloromethane, benzene, toluene, xylene,dimethylbenzene, chlorobenzene and methoxybenzene.

The random nanosilica plates existing in the water phase as obtained instep 3 usually contain no impurity and therefore are suitable forfurther application in modifying polymers.

It has been discovered that the process for direct exfoliation inaccordance with the present invention requires the synthesis of novelexfoliating agents which may be prepared by using the following rawmaterials and chemicals used in experimental examples cited.

-   (1) sodium montmorillonite (Na⁺-MMT), trade name as Kunipia F and    CEC=115 meq/100 g, purchased from Kunimine Industries Co., Ltd.    Japan, or Nanocor and CEC=120 meq/100 g, purchased from Nanocor    Industries Co., Ltd. USA.-   (2) p-cresol, purchased from ACROS Co., Mw 108, m.p. 3134° C., b.p.    202° C.-   (3) polyoxyalkylene diamine starting material

The polyoxyalkylene diamines to be used as starting materials inaccordance with the present invention are poly(propylene glycol)bis(2-aminopropyl ether)s having the formula:

wherein x is a positive number having a value from 2 to 70.

Representative amines have an average molecular weight of about 230wherein the value of x is between 2 and 3 (Jeffamine® D-230), an averagemolecular weight of about 400 wherein the value of x is between 5 to 6(Jeffamine® D-400), an average molecular weight of about 2000 whereinthe value of x is about 33 (Jeffamine® D-2000) and an average molecularweight of about 4000 wherein the value of x is about 68 (Jeffamine®D-4000). All of these polyoxyalkylene diamines are commercial productsand can be purchased from Huntsman Chemical Co. USA, or from AldrichChemical Co. These starting materials are polyoxypropylene-backboned andprimary amine terminated diamines. The poly(propyleneglycol)-block-poly(oxyethylene glycol)-diamines.

-   (4) formaldehyde, purchased from ACROS Co., Mw 30, 37 wt % in    aqueous solution.-   (5) diglycidyl ether of bisphenol-A (BE-188), epoxide equivalent    weight 180, obtained from Chang-Chun Chemical Co. (Taiwan)    Preparation of the Amine-Terminated Mannich Oligomers

In accordance with the present invention, the required exfoliationagents are prepared by the following sequence:

A polyoxypropylene diamine having the composition as described in theabove formula is dissolved in toluene and added with p-cresol at aspecified equivalent ratio. The reaction mixture thus formed is heatedto 30-60° C. and added with formaldehyde at the preferred molar ratio ofpolyoxyalkylene amine, p-cresol and formaldehyde at (n+1):n:2n, wheren=1 to 5, to obtain a polymeric exfoliating agent, amine terminatingMannich oligomer (AMO). The polyoxypropylene diamine is preferred tohave a molecular weight ranging from 2000 to 8,000 g/mol, and theformaldehyde is added at a slow rate to avoid the exothermic temperatureover 120° C.,

In the present invention, the polyoxyalkylene amine preferably hasmolecular weight ranging from 200 to 8,000; and more preferably from 400to 2,000. Polyoxyalkylene amine can be, for example, polyoxypropylenediamine, polyoxyethylene diamine, poly(oxyethylene-oxypropylene)diamine,etc., wherein polyoxypropylene diamine is preferred. A generic chemicalstructure is depicted below, where x=2 to 120 or the molecular weight isin the range of 200 to 8,000.

In the above steps, formaldehyde is preferably added at an equivalentratio ranging from 0.5 to 2.0 molar ratio to amine, whereby more than 25wt % of the total AMO copolymers have molecular weight more than 5,000and less than 10,000 g/mol.

Optionally, the polymeric polyamine exfoliating agent may be of anotherclass of compound, namely the amine terminated epoxy oligomers (AEO),which may be prepared from the epoxy opening reaction of polyoxyalkyleneamine and diglycidyl ether of bisphenol-A at a molar ratio of (m+1): m,where m=1 to 5, a polymeric exfoliating agent, amine terminating Mannicholigomer (AMO), wherein the polyoxyalkylene amine has a molecular weightranging from 200 to 8,000 g/mol and preferably 400 to 2,000 g/mol.

In this invention, by means of cationic exchange reaction between AMO orAEO and sodium montmorillonite, interlayer distances of the clay areenlarged through intercalation and eventually exfoliation to silicaplates in a random manner. However, the ionic bridges between AMO or AEOand individual silica plates still exist in a form of the quaternaryammonium salt/clay complex. In order to separate the exfoliated silicaplates from the organic exfoliating agents, an aqueous solution ofalkaline metal hydroxide such as sodium hydroxide is added into thehybrids to exchange the organic quaternary ammonium cations with sodiumions. The random nanosilica plates can then be isolated in watersuspension. The process requires repetitive displacement reactions forseveral times by adding solvent mixtures of ethanol/water/toluene. Theextraction step requires an appropriate solvent composition which causesthe two-phase separation, with the AMO or AEO amines in the uppertoluene phase and the nanosilica plates suspension in the lower waterphase. For the overall process, the purified nanosilica plate suspensionin water can be obtained and the AMO or AEO exfoliating agent isrecycled.

More detailed procedures of the present invention are described in thepreferred Examples and the Comparative Examples.

EXAMPLE 1 Exfoliation of Na⁺-MMT by Using AMO Agent

Sodium montmorillonite (Na⁺-MMT) (10 g, 11.5 meq) is preliminarilydispersed in water (1 L, 80° C.) in a beaker and swelled by vigorouslystirring for 4 hours to form an earth-colored uniform dispersion beforethe ionic exchange reaction. The following illustrates the experimentalprocedures which are performed in a glass reactor equipped with amechanical stirrer, a thermometer, a condenser, a heating mantle andtemperature controller.

To Prepare the amine-terminating Mannich oligomer (AMO) as theexfoliating agent, to a reactor, p-cresol (13.6 g, 126 mmoles) andpoly(propylene glycol)diamine (Jeffamine D-2000, 378 g, 189 mmole) weredissolved in toluene (200 ml) and the mixture was heated to 90° C. for 3hours. Formaldehyde (37 w % in water, 30 g, 278 mmole) is then graduallyadded in a duration of 4.5 hours, at a rate of about 6-7.5 ml/hour.During the process, the solution temperature was exothermic up to 90°C.-130° C. Stirring continuously for 5 hours, a very viscous product wasobtained. According to the gel permeation chromatography (GPC) analysis,three major peaks at Mw 3,100, Mw 6,200 and Mw 9,200 were observed, onthe basis of polystyrene as the GPC standard. Amine titration values ofthe AMO product are 0.4 meq/g for primary amine and 0.56 meq/g forsecondary amine, and none for tertiary amine, indicating the formationof Mannich secondary amine and the conversion of primary amine into thedesired products.

Step (1): Acidification of the AMO Exfoliating Agent

The AMO product (57.5 g; 23 meq) was not completely soluble in water.With the addition of concentrated hydrochloric acid (35 w % in water,1.2 g; 11.5 meq), the product became soluble or formed an emulsionsolution at 80° C. after being stirred for 30 minutes. The AMOquaternary salt was hence prepared for the MMT exfoliation.

Step (2): Exfoliation of Sodium Montmorillonite Clay

The acidified AMO emulsion (from Step 1) was poured into thepre-prepared dispersion of Na⁺-MMT in water at 80° C. while a vigorousagitation was continued for over 5-hour period. After standing at roomtemperature, the AMO/MMT hybrid was isolated by separating the floatingsolid material from the water phase. A sample of the isolated hybrid wasanalyzed by X-ray diffraction and shown to have none of crystallinephase (exfoliation).

Step (3): Displacement Reaction of AMO Quaternary Ammonium Salt withSodium Ion (I)

To the AMO/MMT hybrid (from Step 2), which was dispersible in tolueneand ethanol but not in water, was directly added an aliqote of aqueousNaOH (4.6 g in water). With agitation, the mixture became acream-colored thick suspension. The solid suspension was filtrated andwashed by mixing with ethanol (750 ml). A second washing procedure wasperformed by adding and stirring with another portion of ethanol (1 L)and filtrated again to obtain a cream-colored translucent AMO/NSP(nanosilica plate) hybrids. The hybrid was analyzed by thermalgravimetric analysis, the result of which indicated an organic (AMO)composition of 40 weight %.

Displacement Reaction of AMO Quaternary Ammonium Salt with Sodium Ion(II)

A second displacement reaction was carried out to remove organic AMOexfoliating agent completely. In this step, the procedure was repeatedby mixing the above AMO/NSP hybrid product with another portion of NaOH(9.2 g) in ethanol (1 L), water (1 L), and toluene (1 L). Aftervigorously stirring and standing for overnight, the mixtures were phaseseparated into an upper toluene phase containing the AMO exfoliatingagent, a middle phase of clear ethanol, and a lower water phasecontaining nanosilica plates (NSP). The AMO copolymers in toluene phasecan be easily isolated by solvent evaporation and recycled.

COMPARATIVE EXAMPLE 1

The above experimental procedures were repeated but using only a halfamount of AMO exfoliating agent prepared from the Mannich reaction ofp-cresol, poly(propylene glycol)diamine and formaldehyde. The resultantAMO/MMT from the Step (3) was analyzed by X-ray diffraction (XRD) whichindicated that the basal spacing was 50 Å, indicative of the MMT in anAMO intercalating mode rather than exfoliation. The result demonstratesthe importance of AMO copolymer amount used for the exfoliation and thepreparation of nanosilica plates.

In Table 1, the interlayer distances (or XRD basal spacing) and weightratios of organic composition of Na⁺-MMT starting material, AMO/MMTexfoliated (Example 1) and intercalated (Comparative Example 1) aresummarized in Table 1.

COMPARATIVE EXAMPLE 2

The experimental procedures of Example 1, Steps 1-3, were repeated butusing a different synthesis of exfoliating agent (abbreviated AMO-1),and repeating the synthesis for the amine-terminating Mannich oligomers(AMO). The Mannich reaction was repeated with the same equivalent ratioof starting materials and reaction temperature, but the formaldehyde wasadded to the p-cresol, poly(propylene glycol)diamine (Jeffamine D-2000)in one-portion rather than dropwise. The resultant AMO-1 was analyzedand the result exhibited a lower molecular weight distribution than thatof AMO in Example 1. The GPC analysis indicated also three peaks at Mw670, Mw 3,000 and Mw 6,000, on the basis of polystyrene standard.

By following the procedures in Step (2), the resultant AMO-1/MMTexhibited a XRD basal spacing (56 Å) rather than an exfoliatingnon-crystalline pattern. By continuing the Step (3), the solid became acream-colored suspension and the AMO-1 polyamine cannot be separatedfrom MMT, since both organics and inorganics were present in the toluenephase. The results indicated that the low molecular weight of AMO-1rendered an intercalated MMT complex rather than an exfoliated MMT. As aconsequence, the inorganic MMT and organic AMO-1 cannot be separated.Hence it is important for AMO to have a suitable molecular weight forexfoliating layered silicate into random silica plates.

COMPARATIVE EXAMPLES 3-5

The experimental procedures of Example 1 were repeated from Step (1) toStep (3). In the step (3), the two phase solvent was toluene/waterwithout the addition of ethanol. It is found the hybrid product can notbe filter due to the high viscous and sticky nature of the product. Itis realized that the proper mixture of solvent including ethanol isrequired to isolate the random silica plates in the process.

EXAMPLE 2 A Larger Scale Experiment

Sodium montmorillonite (Na⁺-MMT 100 g, cationic exchange capacity 115mequiv/100 g) was dispersed into 10 L of hot water (about 80° C.) byusing a homogenizer. Amine terminating-Mannich Oligomer (AMO) wasdissolved in water (575 g) and concentrated hydrochloric acid (35 wt %in water 12 g) were mixed at 80° C. for 30 minutes. The GPC analysis ofhigh Mw AMO used in Example 2 shows three peaks at 4,000, 8,100 and12,600. The solution was poured into the hot aqueous dispersion ofNa⁺-MMT/Water with a vigorous agitation for 5 hours at 80° C. tocomplete the intercalation. The solution becomes phase separated. Repeatprocedures of Example 1, but all the materials are added in quantitiesten time as much. The same nanosilica plates as Example 1 can beeventually obtained. TABLE 1 Exfoliation or intercalation of AMO/clay byusing different acidified ratio and different equivalent amount ofexfoliating agents. Interlayer Equivalent ratio distance Organic/clayweight (NH₂/H⁺/MMT) (Å) ratio ^(a) (w/w %) Comparative 1:1:1 50 72/28Example 1 Comparative 1:3:3 39 45/55 Example 3 Comparative 1:2:2 5057/43 Example 4 Comparative 2:3:3 50 66/34 Example 5 Example 1 2:1:1Exfoliated 83/17^(a) Organic/silicate fraction was determined by TGA at 850° C.

In the Comparative Examples 1, 3, 4 and 5 using the molar ratio of AMOexfoliating agent at one or less than one equivalent to clay CEC, theresults showed the intercalating clay. Only at 2:1 excess amount of thesame AMO, the clay may be exfoliated.

As indicated in the above Examples 1 and 2, the preparation of randomsilica plates can be performed by using an AMO quaternary ammonium saltas the exfoliating agent. The process required several steps includingexfoliation, sodium hydroxide exchange and phase toluene/ethanol/waterextraction. The controlled experiments demonstrated that the AMOmolecular weight, molecular structure, phase extraction solvent andsodium hydroxide equivalent are crucial parameters for effectivelyproducing random silica plates. The nanosilica plates produced in thepresent invention are primary structure and exhibit ionic character andhigh-aspect ratio (surface versus thickness) properties, which aresuitable for improving physical/mechanical properties of polymers, forexample, resistance to solvent, resistance to heat distortion, gasbarrier properties, rigidity, etc.

EXAMPLE 3 Exfoliation of Na⁺-MMT by Using AEO Agent

The following experimental procedures are a typical example for thepreparation of AEO exfoliating agent and its uses for ionic exchangingwith Na⁺-MMT clay.

Preparation of the Amine-Terminating Epoxy Oligomer (AEO) ExfoliatingAgent

To a glass reactor was added poly(propylene glycol)diamine (JeffamineD-2000, 320 g, 160 mmoles) and diglycidyl ether of bisphenol-A (38 g,100 mmoles) dissolved in toluene (500 ml) and the mixture was heated to90° C. for 3 hours. The temperature was slowly increased until allsolvent was removed from a trap condenser. The resulting product was aviscous and yellowish liquid. The polymeric amines were further analyzedby amine titration and GPC (three peaks at 660, 1,900, and 6,000) toconfirm the oligomeric structure. The conversion of primary amines intosecondary amines through the reaction with DGEBA was evidenced by aminetitration, showing 0.36 meq/g for primary amine (theoretical 0.33 meq/g)and 0.67 meq/g for secondary amine (theoretical 0.66 meq/g). Thepolymeric amine was used for the following exfoliation process.

Step (1): Acidification of the AEO Exfoliating Agent

The AEO product prepared in the above synthesis (64 g) was notcompletely soluble in water. With the addition of concentratedhydrochloric acid (35 w % in water, 2.5 g), the product became solubleor formed an emulsion solution at 80° C. after being stirred for 30minutes. The AEO quaternary salt was hence prepared for the MMTexfoliation.

Step (2): Exfoliation of Sodium Montmorillonite Clay

The acidified AEO emulsion obtained from Step (1) was poured into asuspension of Na⁺-MMT (20 g, CEC=120 meq/100 g) in water at 80° C. whilea vigorous agitation was continued for over 5 hour period. After addingethanol (100 ml) and standing at room temperature, the AEO/MMT hybridwas isolated by separating the floating solid material from thewater/ethanol phase. A sample of the isolated hybrid was analyzed byX-ray diffraction and shown to have none of crystalline phase(exfoliation).

Step (3): Displacement Reaction of AEO Quaternary Ammonium Salt withSodium Ion

To the AEO/MMT hybrid obtained in Step (2), which was dispersible intoluene and ethanol but not in water, was directly added an aliquot ofaqueous NaOH (2.4 g in water). With agitation, the mixture became acream-colored thick suspension, to which ethanol (500 ml) was added. Asecond washing procedure was performed by adding and stirring withanother portion of ethanol (500 ml) and filtrated again to obtain acream-colored translucent AEO/NSP (nanosilica plate) hybrids. The secondwashing procedure was repeated by adding another portion of NaOH (2.4 gin 25 ml water), ethanol (300 ml) and toluene (150 ml). The mixture wasagitated and stood for phase separation. A three-layer suspension wasobtained and the bottom water layer contained nanosilica plates.

1. A method for producing random form of nanosilica plates, comprisingthe steps of: (a) preparing amine-terminating Mannich oligomers (AMO)from polyoxyalkylene amine, p-cresol and formaldehyde to obtain apolymeric exfoliating agent having a general formula:

wherein n is from 1 to 5; R represents an organic group selected fromthe group consisting of polyoxypropylene groups,polyoxyethylene/oxypropylene groups, and polyethylene amino groups;wherein said formaldehyde is added at an appropriate addition rate bywhich the exothermic temperature is kept below 120° C.; (b) addinginorganic acid to acidify said AMO; (c) mixing said acidified AMO with aswelled inorganic layered silicate clay so as to exfoliate said silicateclay directly through cationic exchange; (d) adding an aqueous solutioncontaining a hydroxide or a chloride of alkali metal or alkaline-earthmetal, ethanol, water and an organic solvent to said exfoliated silicateclay obtained in step (c), and then keeping the mixture static to forman upper organic phase and a lower water phase containing nanosilicaplates.
 2. The method as claimed in claim 1, wherein saidpolyoxyalkylene amine used in said step (a) has molecular weight rangingfrom 400 to 4,000 g/mol.
 3. The method as claimed in claim 1, whereinsaid polyoxyalkylene amine used in said step (a) has molecular weightranging from 1,000 to 2,000 g/mol.
 4. The method as claimed in claim 1,wherein said polyoxyalkylene amine used in said step (a) is selectedfrom the group consisting of polyoxypropylene diamine, polyoxyethylenediamine, polyoxybutylene diamine andpoly(oxyethylene-oxypropylene)diamine polyetheramines.
 5. The method asclaimed in claim 1, wherein said polyoxyalkylene amine used in said step(a) is polyoxypropylene diamine.
 6. The method as claimed in claim 1,wherein said polyoxyalkylene diamine, p-cresol and formaldehyde in saidstep (a) are added at a molar ratio (n+1):n:2n, wherein n is 1 to
 5. 7.The method as claimed in claim 1, wherein said formaldehyde used in saidstep (a) is added at a reaction temperature in the range of 25° C. to160° C.
 8. The method as claimed in claim 1, wherein 25˜100 wt % of saidexfoliating agent obtained in said step (a) has molecular weight rangingfrom 9,000 to 20,000.
 9. The method as claimed in claim 1, wherein saidexfoliating agent and said inorganic acid are mixed in an equivalentratio 2:1 in said step (b).
 10. The method as claimed in claim 1,wherein said inorganic acid used in said step (b) is selected from thegroup consisting of hydrochloric acid, sulfuric acid, phosphoric acidand nitric acid.
 11. The method as claimed in claim 1, wherein the molarratio of amino groups in said exfoliating agent to cationic exchangecapacity of said silicate clay ranges from 3:1 to 1:1 in said step (c).12. The method as claimed in claim 1, wherein said silicate clay used insaid step (c) is selected from the group consisting of montmorillonite,kaolin, mica and talc.
 13. The method as claimed in claim 1, whereinsaid silicate clay used in said step (c) has a cationic exchangecapacity ranging from 50 meq/10 g to 200 meq/100 g.
 14. The method asclaimed in claim 1, wherein said hydroxide or chloride of alkali metalor alkaline-earth metal used in said step (d) is sodium hydroxide. 15.The method as claimed in claim 1, wherein said hydroxide or chloride ofalkali metal or alkaline-earth metal is added at the same equivalence insaid step (d).
 16. The method as claimed in claim 1, wherein saidorganic solvent used in said step (d) is selected from the groupconsisting of ether, ketone, ester, nitrile, saturated hydrocarbon,chlorinated saturated hydrocarbon and aromatic hydrocarbon.
 17. Themethod as claimed in claim 1, wherein said organic solvent used in saidstep (d) is selected from the group consisting of tetrahydrofuran,isopropyl ether, methyl tert-butyl ether, methyl isobutyl ketone,ethylnitrile, ethyl acetate, pentane, hexane, heptane, cyclohexane,dichloromethane, benzene, toluene, dimethylbenzene, chlorobenzene andmethoxybenzene.
 18. The method as claimed in claim 1, wherein saidhydroxide or chloride of alkali metal or alkaline-earth metal is addedat 1 to 5 equivalences in step (d).
 19. The method as claimed in claim1, wherein said organic phase formed in said step (d) comprises saidexfoliating agent suitable for recycling.
 20. A nanosilica plate, whichis obtained in water suspension with solid content from 0.1 to 90 weightpercent in said step (d) of the method as claimed in claim
 1. 21. Amethod for producing random form of nanosilica plates, comprising thesteps of: (a) preparing amine-terminating epoxy oligomers (AEO) frompolyoxyalkylene amine and diglycidyl ether of bisphenol-A at a molarratio of (m+1): m, where m—1 to 5, and at a reaction temperature between25° C. and 150° C., to obtain a polymeric amine exfoliating agent,having a general formula:

wherein m is from 1 to 5; R is an organic group selected from the groupconsisting of polyoxypropylene groups, polyoxyethylene/oxypropylenegroups, and polyethylene amino groups; (b) adding inorganic acidincluding hydrochloric acid to acidify said AEO; (c) mixing saidacidified AEO with a swelled inorganic layered silicate clay so as toexfoliate said silicate clay directly through cationic exchange; (d)adding an aqueous solution containing a hydroxide or a chloride ofalkali metal or alkaline-earth metal, ethanol, water and an organicsolvent to said exfoliated silicate clay obtained in step (c), and thenkeeping the mixture static to form an upper organic phase and a lowerwater phase containing nanosilica plates.
 22. The method as claimed inclaim 21, wherein said polyoxyalkylene amine used in said step (a) hasmolecular weight ranging from 400 to 4,000 g/mol.
 23. The method asclaimed in claim 21, wherein said polyoxyalkylene amine used in saidstep (a) is selected from the group consisting of polyoxypropylenediamine, polyoxyethylene diamine, polyoxybutylene diamine andpoly(oxyethylene-oxypropylene)diamine polyetheramines.
 24. The method asclaimed in claim 21, wherein 25-100 wt % of said exfoliating agent insaid step (a) has molecular weight ranging from 2,000 to 20,000.
 25. Themethod as claimed in claim 21, wherein said exfoliating agent and saidinorganic acid are mixed in an equivalent ratio 2:1 in said step (b).26. The method as claimed in claim 21, wherein the molar ratio of aminogroups in said exfoliating agent to cationic exchange capacity of saidsilicate clay ranging from 3:1 to 1:1 in said step (c).
 27. The methodas claimed in claim 21, wherein said silicate clay used in said step (c)is selected from the group consisting of montmorillonite, kaolin, andmica.